Network Load Balancer - Disconnecting Interfaces

After reading countless guides on Microsoft Network Load Balancer and none of them making any sense I eventually came across how this should actually work.

Configuring NLB is straight forward. Understanding what is required from the networking was where it got confusing for me, as no one really explained this properly.

For NLB to work, you need 2 network interfaces. 1 is your servers IP address that it is accessed from by other servers / clients on your network. The second is the NLB network, which us used purely for communication between the balanced servers.

The network details:

Actual Server IP range: 10.0.0.0 /24 (255.255.255.0)
Cluster IP Required: 10.0.0.10 /24
NLB IP range: 10.0.1.0 /24

The network that that the 2 interfaces are on MUST be the same. So in VMWare or even physical they should be on the same VLAN. And if both are in the same subnet they should be able to communicate. I will explain more in a sec.

Now give your networks in Windows some names to make them easier to see.

Configure your LAN IP's as you would normally. Once that’s done assign your NLB network an IP address in its range.

So in my example range above your LAN IP would be something like 10.0.0.1 /24. Give your NLB an IP that makes sense, so 10.0.1.1 /24 would be good. Do the same on the next server(s).

Do not set a default gateway or any DNS on the NLB interface.

If you have NLB installed, great. If not... install it. Create your cluster as everyone recommends. Selecting your NLB interface as the dedicated IP, set your cluster address on your server IP range and do the port stuff as you need it.

What I found with my struggles was that if you have the NLB interface on a separate VLAN or network I couldn’t get to the cluster IP from my Server network. If I put the NLB interface onto the Server IP range the 2 interfaces would have IP conflicts and only 1 would work.

This way, there is no conflict. The Cluster IP address belongs to the NLB cluster on your Server IP range and not the interfaces that are on that range. On the NLB network the IP address for the Cluster isn’t accessible from the NLB interfaces so they don’t get affected by the conflict.

Hope this helps anyone scratching their head wondering why NLB isn’t working.

Instructions on how to configure NLB can be found here:

http://technet.microsoft.com/en-us/library/cc754071(v=ws.10).aspx